1. Technical Field
In one aspect, this invention relates to methods of reducing catalyst attrition losses for hydrocarbon synthesis processes conducted in high agitation reaction systems. More particularly, but not by way of limitation, the present invention relates to methods of reducing catalyst attrition losses for hydrocarbon synthesis processes conducted in three-phase reaction systems. In another aspect, this invention relates generally to attrition resistant catalysts for conducting Fischer-Tropsch synthesis.
2. Background
In Fischer-Tropsch processes, synthesis gases comprising carbon oxides and hydrogen arc reacted in the presence of Fischer-Tropsch catalysts to produce liquid hydrocarbons. Fischer-Tropsch synthesis processes are most commonly conducted in fixed bed, gas-solid or gas-entrained fluidized bed reaction systems, fixed bed reaction systems being the most commonly used. It is recognized in the art, however, that slurry bubble column reactor systems offer tremendous potential benefits over these commonly used Fischer-Tropsch reaction systems. However, the commercial viability of slurry bubble column processes has been questioned. The unique reaction conditions experienced in slurry bubble column processes are extremely harsh. Thus, catalyst attrition losses in slurry bubble column processes can be both very high and costly. In fact, many of the best performing catalysts employed in other Fischer-Tropsch reaction systems quickly break down when used in slurry bubble column systems.
Heretofore, little has been done to even evaluate or model the harsh conditions experienced in slurry bubble column reactor processors, much less solve the attrition loss problem. Thus, a need presently exists for a means of both (a) reducing catalyst attrition losses and (b) increasing the viability of higher performance catalysts in slurry bubble column processes and in other such "high agitation" reaction systems.
As mentioned above, the synthesis gas, or "syngas," used in Fischer-Tropsch processes is typically a mixture consisting primarily of hydrogen and carbon oxides. Syngas is typically produced, for example, during coal gasification. Processes are also well known for obtaining syngas from other hydrocarbons, including natural gas. U.S. Pat. No. 4,423,265 to Chu et al. notes that the major processes for producing syngas depend either upon the partial combustion of a hydrocarbon fuel with an oxygen-containing gas or the reaction of the fuel with steam, or on a combination of these two reactions. U.S. Pat. No. 5,324,335 to Benham et al., explains the two primary methods (i.e., steam reforming and partial oxidation) for producing syngas from methane. The Encyclopedia of Chemical Technology, Second Edition, Volume 10, pages 3553-433 (1966), Interscience Publishers, New York, N.Y. and Third Edition, Volume 11, pages 410-446 (1980), John Wiley and Sons, New York, N.Y. is said by Chu et al. to contain an excellent summary of gas manufacture, including the manufacture of synthesis gas.
It has long been recognized that syngas can be converted to liquid hydrocarbons by the catalytic hydrogenation of carbon monoxide. The general chemistry of the Fischer-Tropsch synthesis process is as follows: EQU CO+2H.sub.2.fwdarw.(--CH.sub.2 --)+H.sub.2 O (1) EQU 2CO+H.sub.2.fwdarw.(--CH.sub.2 --)+CO.sub.2 (2)
The types and amounts of reaction products, i.e., the lengths of carbon chains, obtained via Fischer-Tropsch synthesis vary dependent upon process kinetics and the catalyst selected.
Many attempts at providing active catalysts for selectively converting syngas to liquid hydrocarbons have previously been disclosed. U.S. Pat. No. 5,248,701 to Soled et al., presents an over-view of relevant prior art. The two most popular types of catalysts heretofore used in Fischer-Tropsch synthesis have been iron-based catalysts and cobalt-based catalysts. U.S. Pat. No. 5,324,335 to Benham et al. discusses the fact that iron-based catalysts, due to their high water gas shift activity, favor the overall reaction shown in (2) above, while cobalt-based catalysts tend to favor reaction scheme (1).
Recent advances have provided a number of catalysts active in Fischer-Tropsch synthesis. Besides iron and cobalt, other Group VIII metals, particularly ruthenium, are known Fischer-Tropsch catalysts. The current practice is to support such catalysts on porous, inorganic refractory oxides. Particularly preferred supports include silica, alumina, silica-alumina, and titania. In addition, other refractory oxides selected from Groups III, IV, V, VI and VIII may be used as catalyst supports.
The prevailing practice is to also add promoters to the supported catalyst. Promoters can include ruthenium (when not used as the primary catalyst component), rhenium, hafnium, cerium, and zirconium. Promoters are known to increase the activity of the catalyst, sometimes rendering the catalyst three to four times as active as its unpromoted counterpart.
Contemporary cobalt catalysts are typically prepared by impregnating the support with the catalytic material. As described in U.S. Pat. No. 5,252,613 to Chang et al., a typical catalyst preparation may involve impregnation, by incipient wetness or other known techniques, of, for example, a cobalt nitrate salt onto a titania, silica or alumina support, optionally followed or preceded by impregnation with a promoter material. Excess liquid is then removed and the catalyst precursor is dried. Following drying, or as a continuation thereof, the catalyst is calcined to convert the salt or compound to its corresponding oxide(s). The oxide is then reduced by treatment with hydrogen, or a hydrogen-containing gas, for a period of time sufficient to substantially reduce the oxide to the elemental or catalytic form of the metal. U.S. Pat. No. 5,498,638 to Long points to U.S. Pat. Nos. 4,673,993, 4,717,702, 4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, and 5,292,705 as disclosing well known catalyst preparation techniques.
As also mentioned above, Fischer-Tropsch synthesis has heretofore been conducted primarily in fixed bed reactors, gas-solid reactors, and gas-entrained fluidized bed reactors, fixed bed reactors being the most utilized. U.S. Pat. No. 4,670,472 to Dyer et al. provides a bibliography of several references describing these systems. The entire disclosure of U.S. Pat. No. 4,670,472 is incorporated herein by reference.
In contrast to these other hydrocarbon synthesis systems, slurry bubble column reactors are "three phase" (i.e., solid, liquid, and gas/vapor) reaction systems involving the introduction of a fluidizing gas into a reactor containing catalyst particles slurried in a hydrocarbon liquid. The catalyst particles are slurried in the liquid hydrocarbons within a reactor chamber, typically a tall column. Syngas is then introduced at the bottom of the column through a distributor plate, which produces small gas bubbles. The gas bubbles migrate up and through the column, causing beneficial agitation and turbulence, while reacting in the presence of the catalyst to produce liquid and gaseous hydrocarbon products. Gaseous products are captured at the top of the SBCR, while liquid products are recovered through a filter which separates the liquid hydrocarbons from the catalyst fines. U.S. Pat. Nos. 4,684,756, 4,788,222, 5,157,054, 5,348,982, and 5,527,473 reference this type of system and provide citations to pertinent patent and literature art. The entire disclosure of each of these patents is incorporated herein by reference.
It is recognized that conducting Fischer-Tropsch synthesis using a SBCR system could provide significant advantages. As noted by Rice et al. in U.S. Pat. No. 4,788,222, the potential benefits of a slurry process over a fixed bed process include better control of the exothermic heat produced by the Fischer-Tropsch reactions, as well as better maintenance of catalyst activity by allowing continuous recycling, recovery and rejuvenation procedures to be implemented. U.S. Pat. Nos. 5,157,054, 5,348,982, and 5,527,473 also discuss advantages of the SBCR process.
Although the use of slurry bubble column reactors for commercial applications offers significant potential advantages over fixed bed and other types of reactor systems, the viability of the slurry bubble column process has heretofore been questioned, owing in large part to high catalyst attrition losses and costs. As mentioned above, slurry bubble column reactor processes are extremely demanding on catalysts from a physical strength standpoint. Many catalyst formulations lack any practical application in SBCR's because of the rate of physical attrition experienced. In addition to catalyst loss, the physical destruction and attrition of the catalyst results in (a) poorer distribution of the catalyst in the reactor, (b) filtration problems in removing liquid products, and (c) possible contamination of the products with catalytic material.
The significance of the attrition problem was seen, for example, during the Fischer-Tropsch Demonstration Run III conducted in October 1996 at the U.S. Department of Energy's Alternative Fuels Development Unit (a slurry bubble column reactor) in LaPorte, Tex. (See Brown et al., in Paper 27E for AICHEME Meeting in Houston, Mar. 10, 1997). The catalyst selected for that demonstration was a promising, "improved" cobalt catalyst which exhibited high activity in laboratory tests. However, the LaPorte run had to be terminated when the catalyst unexpectedly broke down and seriously plugged the process filters.
As this example also suggests, most of the work performed heretofore in Fischer-Tropsch catalyst development has focused on the activity and/or selectivity of the catalysts, with little or no attention being given to their physical or mechanical properties. Most catalysts have been designed for fixed bed reaction systems, which are much less demanding in terms of attrition resistance than are slurry bubble column reactors.
Recently, U.S. Pat. Nos. 5,648,312, 5,677,257, and 5,710,093 disclosed formulations of hydrogenation catalysts which are said to provide improved attrition resistance. The catalyst supports used in these formulations arc substantially spherical particles consisting of substantially homogeneous mixtures of silica particles and silicon carbide particles.
It is known that the use of spheroidal supports in the preparation of supported metal catalysts for fluidized bed applications tends to reduce catalyst attrition. However, the mere use of spherical supports is not sufficient, in and of itself, to obtain acceptable attrition resistance for slurry bubble column applications.